1. Field of the Invention
This invention generally relates to a bearing assembly for providing a linear relative motion between two elements, and, in particular, to a linear motion rolling contact bearing assembly employing a plurality of rollers in an endless circulating path.
2. Description of the Prior Art
A linear motion rolling contact bearing assembly for providing a linear relative motion between two elements is well known in the art. Typically, such a linear motion bearing assembly includes a rail extending straight over a desired distance, theoretically over an indefinite length, and a casing or slider which moves along the rail. An endless circulating path is defined in the slider and partly between the slider and the rail, and a plurality of rollers are provided in the endless circulating path, thereby providing a rolling contact between the slider and the rail. Typically, the endless circulating path has a cross section generally square in shape and the rollers are arranged with their axes oriented perpendicular to each other alternately. The endless circulating path includes a pair of straight load and return sections and a pair of circular connecting sections, each connecting the corresponding ends of the load and return sections. The straight load section is normally defined between the rail and the slider and the straight return section is defined within the slider.
In such a linear motion rolling contact bearing assembly, in order to improve the smoothness in movement of the rollers in particular through each of the circular connecting sections of the endless circulating path, it has been proposed to use rollers each of which is generally cylindrical in shape and has a pair of end surfaces, one of which is flat and the other of which is arcuate or dome-shaped, as disclosed in the Japanese Utility Model Application No. 60-19057, which is hereby incorporated by reference. For the sake of convenience, the flat end surface of this cylindrical roller will be referred to as an F end surface and the arcuate or dome-shaped end surface of the cylindrical roller will be referred to as an A end surface. Thus, a cylindrical roller having a flat end surface and an arcuate or dome-shaped end surface will be referred to as an A-F roller. In accordance with the teachings of the above-identified Japanese Utility Model Application No. 60-19057, A-F rollers are arranged in the endless circulating path such that the A-F rollers rollingly move along each of the circular connecting portions with their A end surfaces oriented radially outwardly and their F end surfaces oriented radially inwardly.
It is true that the arrangement disclosed in the above-identified Japanese Utility Model Application No. 60-19057 improves the smoothness in the sliding movement of the rollers especially through each of the curved connecting portions of the endless circulating path. However, because of tolerances in providing an arcuate or dome-shaped roller, the height could differ from roller to roller when manufactured, and this could increase the sliding resistance of the A-F rollers against their rolling movement along the endless circulating path. Since the rollers are different in height, shorter rollers tend to skew between the two adjacent longer rollers, which could cause an increase in the sliding resistance and instability in operation.